The invention relates to a method of optically writing and reading information in the form of a pattern of magnetic domains in an information layer of a record carrier. An optical scanning beam is focused to a radiation spot and during writing the portions of the information layer heated by the radiation spot are subjected to the influence of a magnetic field which is directed substantially perpendicularly to the information layer, which field is generated by means of a coil through which an energizing current is passed, so that said domains are formed. During reading, the variation caused by the magnetic domains in the state of polarization of the scanning beam is detected.
The invention also relates to an apparatus for performing the method.
Such a method of recording information is known from U.S. Pat. No. 4,466,004, issued Aug. 14, 1984. Generally a laser beam, for example a diode laser beam, which is focused to a diffraction-limited radiation spot by means of an optical system, is used when inscribing a magneto-optical information layer. The original uninscribed information layer is premagnetized in a direction perpendicular to this layer. During writing the portion of the magneto-optical material at the location of the radiation spot is heated to a given temperature, for example, the Curie temperature, so that the coercive force is locally reduced. As a result, this portion can be magnetized by a relatively small external magnetic field in a desired direction perpendicular to the magnetic layer. After the relevant portion of the magneto-optical information layer has cooled down, the magnetic direction of the external magnetic field becomes frozen, as it were, into the information layer. By moving the radiation spot and the record carrier with respect to each other and by modulating the external magnetic field, a series of magnetic domains, or information areas, having a direction of magnetization deviating from their surroundings can be written in the information layer, the successive information areas in the direction of movement representing the inscribed information.
This method is known as the magnetic field modulation method. It is alternatively possible to inscribe magnetic domains by means of a constant external magnetic field and by modulating the intensity of the radiation beam in accordance with the information to be written. In this so-called radiation source modulation method the size of the information areas is determined by the size of the radiation spot. In known systems, in which the radiation spot has a half-value width of approximately 1 .mu.m, the information areas are substantially circular with a diameter of the order of 1 .mu.m. The information density is then of the order of 10.sup.6 bits per mm.sup.2.
There is an ever increasing need for larger information densities so that more information can be stored in a record carrier of equal dimensions. To this end it must be possible to write and read information areas which are smaller than those hitherto used in a magneto-optical record carrier.
In said U.S. Pat. No. 4,466,004 it is proposed to provide information areas in the form of magnetic domains in a magneto-optical record carrier, which areas have a dimension in the scanning direction which is smaller than the dimension of the write-radiation spot, by switching the magnetic field at a high frequency. Firstly, the area of the information layer under the radiation spot is magnetized in a direction opposite to the original direction of magnetization of the information layer. Then, while the radiation spot is still partly above said area, the magnetic field is reversed so that said part of the area acquires the original direction of magnetization again. U.S. Pat. No. 4,466,004 does not state how the magnetic domains with their smaller dimension in the scanning direction thus obtained can be read.
Since each information bit is fixed in an information area, each information area must be read separately. This means that reading must be performed by means of a radiation spot whose dimension in the scanning direction is of the same order as the dimension of each of the information areas. The read-radiation spot must therefore be considerably smaller than the write-radiation spot.
An optical scanning system in which radiation at a given wavelength, .lambda., and an objective lens having a given numerical aperture, NA, are used, has an optical cut-off frequency f.sub.co which is proportional to 2.NA/.lambda., i.e. inversely proportional to the size of the scanning spot. Such a system can no longer separately observe details of an object, in this case the information areas in the information layer, if the mutual distance between these details or areas is equal to or smaller than 2.NA/.lambda.. Thus, a given spatial frequency f.sub.r of the information areas is associated with this optical cut-off frequency.
Since the size of the diffraction-limited radiation spot is proportional to .lambda./NA, in which .lambda. is the wavelength of the radiation used and NA is the numerical aperture of the objective system used, the radiation spot can only be reduced by decreasing the wavelength and/or enlarging the numerical aperture. An enlargement of the numerical aperture involves a decrease of the depth of field of the radiation beam so that the requirements to be imposed on the focusing of the radiation beam become more stringent. Moreover, an objective system having a larger numerical aperture is more sensitive to aberrations so that stricter tolerance requirements must be imposed on the write-read apparatus. If a diode laser is to be maintained as a radiation source, which is necessary in a mass product which the magneto-optical write-read apparatus envisages to be, the reduction of the wavelength of the radiation beam is not a real possibility because there are no short-wavelength diode lasers which yield a sufficiently high power for writing.